Victor Penner

Please don't hate me for asking a question that probably has an obvious answer if you are a techie (which I am not).

If I did the stock overbore for a b18c5 which is 0.25 mm or 0.010 in. and did not mill my head or change the shape of the dome at all and used the stock dome shape overbore piston, would my compression ratio change higher, lower or stay the same? My thought is that it would go higher but I am confused.

Answers with explanations would be great.

Thanks,

Victor Penner

Kataku2K3

Yes your compression ratio will go up slightly along with your displacement... Right now your motor is 1797cc and has a C/R of 10.60:1 going .25mm over would bump you to 1809cc and 10.65:1.. hehehe not a big difference but yea that's what happens.

Kataku2K3

Sorry, this happens because there is an extra .25mm of bore which in tern will suck in more air on the downstroke of the piston and which gets smashed back up and compressed.. Think of a syringe that is wider.... When you suck up liquid or solution into it you get more (the solution would be the air) Then if you were able to cap the syringe off and smash it back up (which you can't do with liquid) there would be getting more smashed then a syringe that is smaller. Hope you can understand this. Also remember that the stroke stays the same... Just as you didn't change your crank to alter the stroke the syringe would have the same stroke, just a difference in bore. Latez

Turbo Lude SH

So basically your wondering if running your overbore pistons will effect your compression ratio?

Your compression ratio would actually increase with going to the overbore, but were talking about .05.

So if your stock compression was 10.0 and you went to .25 overbore, your compression afterwards would be 10.05. (+/- .01)

Victor Penner

Thanks for the (kind) replies.

This is why I am confused. You have explained that the larger bore has a greater volume of air which is still compressed by the same stroke length hence the ratio increase (minimal I know, and thanks for the actual #'s). In a turbo application people will substitute flat top pistons to lower their compression ratio. The flat top also increases the volume of air in the cylinder but the compression ratio is lower. Is it because the flat top is effectively shortening TDC?

Sorry...I'm almost there..

Victor Penner

Kataku2K3

It's all good man, the only way we learn is going over ****.... When you have a piston with positive cc of dome or even negative cc of dome, it does draw in more air (volume) but the piston to deck height has been modified. This in turn will change your compression.. Say for instance that you tossed some std. bore USDM GSR pistons in a ITR motor... It would drop your compression down to around 9.8:1, that's with a piston that has -0.60cc of dome, where as the USDM ITR has 3.64cc's of postive dome... Hope this helps

Victor Penner

Thanks!

I get it...finally So do the Spoon or Mugen 2 layer gaskets reall raise the compression ratio as much as they claim? From 10.6 to 1 to 11.? to 1?

Victor Penner

Kataku2K3

Yea they really do make a difference... I don't do this but this is what happens... A stock head gasket is around 0.029" for every layer you remove (0.010") you'll get around 0.27-0.28:1 bump up in compression on a standard 81mm bore.. Likewise if you had a 1 layer head gasket around 0.009" thick you could bump up your compression arond 0.55:1.. Like I said I don't like takin apart OEM headgaskets because they generally won't seal as well... Cometic makes good aftermarket headgaskets for any application... boost/high compression..


[Modified by JDMspecEG6, 11:52 PM 11/12/2002]

Cow

Hey, I have a semi-related question: would it be worth it to enlarge the combustion chamber if one were just doing a .25 overbore? I know you'd want to for 84mm but I figure you could get away with stock cc for 81.25. Just wondering if there would be any significant flow impairments created by the diameter differences..?

Kataku2K3

I wouldn't bother for just going .25mm over, really i don't know if it will cause any flow impairments. You should be just fine with that little tiny increase in bore.. Peace

QCKGSR

10.63 :1 ok

Victor Penner

Thanks again for the reply's. The reason I asked is because I am racing my 97R in SCCA Touring 2 (T2) class which is essentially a showroom stock class. I am allowed to balance and blueprint to factory service limits and I was wondering if this legal overbore would be worth doing this winter. The car has been great and I think that after I do a compression check and a leak down test I will make a decision. What do you think?

Victor Penner

Kataku2K3

Yea that would be cool, see how your motor is now, then make your final desicion!

Turbo Lude SH

Like said their isnt really a need to enlarge the combustion chamber when only going to .25mm overbore. Say you were going 2mm overbore then it would be a good idea. Not only for head gasket issues but also will increase the effeciency in the engine. It wouldn't be a big difference, but worth the effort.

I went form a 87mm piston to a 90mm and I had my combustion chambers enlarged to match. If you want to do it, any good head work place like Portflow will do it for you as long as you supply the headgasket which will match your overbore.